High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGTs) allow the intake airflow to be optimized over a range of engine speeds. This may be accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
A particular problem with turbocharged diesel engines is poor acceleration, particularly from idle or low engine speeds. This "turbo-lag" is due to the time delay associated with filling the intake manifold with enough fresh air to support the amount of fuel required to satisfy the operator's torque demand. To meet this requirement, however, the delivered fuel often must be limited as a function of the available air in order to maintain the air-to-fuel ratio above the threshold at which visible smoke occurs. The rate at which the air supply can be increased is limited by the dynamics of the turbocharger and the transport delay between the turbocharger compressor and the intake manifold of the engine.
A traditional control strategy for diesel engines having an EGR system and a VGT is two single loop controllers. In other words, the two devices are controlled independently with the EGR valve controlling the mass of airflow into the intake manifold (MAF), and the VGT controlling the intake manifold pressure (MAP, or boost). The desired values for compressor mass airflow (MAF) and boost pressure (MAP) are stored as lookup table values referenced to engine speed and load or fueling rate. For each engine speed and fueling rate, the control algorithm retrieves the desired values for MAP and MAF and controls the EGR and VGT to achieve those values.
The two single loop controllers can be represented as follows: EQU EGR=K.sub.EGR (MAF-MAF.sub.d) EQU VGT=K.sub.VGT (MAP-MAP.sub.d)
wherein the subscript "d" denote the desired setpoints for the given variable. The controller K is usually a proportional-integral-derivative (PID) controller, with gains scheduled on the speed-load condition of the engine.
The desired setpoints for MAP and MAF are typically optimized for steady-state engine operation. Because these values were optimized for steady-state engine operation, however, they are poorly suited for generating feedback errors to drive the EGR and VGT during transient conditions. This straightforward engine control strategy often results in excessively large turbo lag and slow engine torque response.
To improve acceleration, some engine control systems use a transient detection feature to turn off the feedback control to the EGR and close the EGR valve when fuel limiting is active. This is done to provide as much fresh air as possible to the intake manifold so that the maximum amount of fuel can be injected without violating the air/fuel threshold at which visible smoke occurs. The independent feedback control for MAF is then reinitiated after the transient condition is over and the engine is essentially operating at steady-state. Such engine control strategies fail to account for the interaction between the VGT and EGR, however, because of their independent control of the two systems.
Thus, there exists a need to control the EGR and VGT and, hence, MAF and MAP, to deliver fuel to the engine at a rate which generates the torque demanded by the driver, yet maintains the air/fuel ratio above the threshold at which visible smoke occurs.